X Rays, electromagnetic radiation similar to that of visible light, but of much shorter wavelength. Virtually all matter is, to some degree, transparent to X rays. For this reason, X rays make it possible to see through flesh, bones, metals, and other substances through which visible light cannot pass. X rays are an invaluable tool in medicine, technology, and scientific research.
X rays are sometimes called Roentgen rays after Wilhelm Konrad Roentgen, who discovered them in 1895. Roentgen called the phenomenon X rays because the symbol X stands for the unknown, and he was unable to determine the nature of the radiation. Not until 1912 was it learned that X rays are electromagnetic radiation. X rays are measured in units called rads and roentgens. A physician who specializes in the diagnosis and treatment of disease by X rays is called a radiologist or a roentgenologist.
